1. Field of the Invention
The present invention is directed to a device for biological tissue stimulation, in particular an implantable device such as a heart pacemaker.
2. Description of the Prior Art
Implantable devices for biological tissue stimulation are known which include a stimulation pulse generator which generates the stimulation pulses, and a detector which determines the reaction of the tissue to the stimulation pulse. A control unit is included which controls the stimulation pulse generator and which identifies the stimulation sensitivity of the tissue. The control unit sets the energy level of the stimulation pulses at a level just high enough to evoke a reaction in the stimulated tissue, by continually reducing the energy of the stimulation pulses, starting from a value lying above the stimulation sensitivity, until there is no reaction of the tissue to a stimulation pulse.
A device of this type is described in German AS 22 54 928, corresponding to U.S. Pat. No. 3,757,792, in the context of a heart pacemaker by which the heart of a patient is stimulated by the stimulation pulses. It is known that a contraction of the heart muscle, as a reaction of the heart to a stimulation pulse, occurs only when the energy of the stimulation pulse exceeds a defined stimulation sensitivity, or stimulation threshold, of the heart tissue. This stimulation sensitivity will vary from patient to patient. To maintain the energy consumption of the heart pacemaker low, instead of using a stimulation energy at a level which is a fixed, constant value above the current stimulation sensitivity, the energy of successive stimulation pulses is reduced in steps, and the reaction or response of the heart to each stimulation pulse is monitored. When no reaction occurs, the stimulation energy is then increased to a value known to lie above the stimulation sensitivity of the heart, and the same procedure is repeated. A matching of the stimulation energy, and thus of the energy consumption within the heart pacemaker, is thereby accomplished relative to the variable stimulation sensitivity of the heart.
In a similar arrangement for cardiac stimulation disclosed in European Application 0 236 562, the stimulation energy following the detection of a tissue reaction after a stimulation pulse is always diminished by a defined value, whenever a cardiac reaction was detected after each of the two immediately preceding stimulation events. If no heart reaction is detected to the current stimulation pulse, the heart is stimulated with a stimulation pulse having maximum energy, and the next stimulation pulse has an energy content increased by a defined amount in comparison to the energy of the unsuccessful stimulation pulse.